This invention addresses a method for adjusting differences in data transmission rates such as between data terminal equipment (DTE) and a modem by stop character manipulation in an asychronous data stream.
When a synchronous DTE sends data to an asynchronous modem at a rate slightly higher than the modem transmission rate, compensation must be provided by the modem or data will be lost. The CCITT V.22 recommendation provides a set of rules which permits certain stop bits to be deleted in order to compensate to a limited overspeed data condition from a DTE.
It is understood by those skilled in the art that asynchronous modem communications utilizes start and stop bits which sandwich character bits being transmitted. The start and stop bits are utilized in the modem transmission network to define the beginning and end of characters. Following the stop bit that indicates the end of a character, there may be additional stop bits before the start bit at the beginning of the next character. As long as both the sending and receiving modem follow compatible rules, it is possible for the sending modem to delete certain information such as stop bits in order to accommodate data being received at a higher rate than can be accommodated by the originating modem.
For example, the CCITT V.22 recommendation allows a stop bit to be omitted by the transmitting modem up to once every 4 (or 8) frames. The receiving modem inserts a shortened stop bit of 3/4 (or 7/8) for the stop bit not transmitted and shortens the period of the next 3 (or 7) stop bits 1/4 (or 1/8) to regain time synchronization with the received frames.
The CCITT V.14 recommendation provides rules for conveying start-stop characters over synchronous bearer channels using an async-to sync converter. This method is intended to replace the earlier recommendation V.22. While the V.14 recommendation provides certain rules and guidelines, there exists a need for an improved overspeed handling method which is consistent with this recommendation and the note in section 7.2 of the recommendation.
It is an object of the present invention to provide an improved overspeed handling method consistent with the guidelines established by CCITT recommendation V.14.